Plasma processing method and apparatus with control of plasma excitation power

ABSTRACT

The amount of RF power supplied to a plasma in a vacuum plasma processing chamber is gradually changed on a preprogrammed basis in response to signals stored in a computer memory. The computer memory stores signals so that other processing chamber parameters (pressure, gas species and gas flow rates) remain constant while the gradual change occurs. The stored signals enable rounded corners, instead of sharp edges, to be etched, e.g., at an intersection of a trench wall and base.

FIELD OF INVENTION

[0001] The present invention relates generally to vacuum plasmaprocessors for processing workpieces on a workpiece holder and moreparticularly to a method of and apparatus for gradually changing, on apreprogrammed basis, power an AC plasma excitation source supplies toplasma in a vacuum processor chamber.

BACKGROUND ART

[0002] Vacuum processors for processing a workpiece (i.e., etchingmaterials from or depositing materials onto the workpiece) typicallyinclude first and second ports respectively connected to a vacuum pumpand one or more gas sources. The gas is excited to a plasma in thechamber by an electric source including a reactance responsive to afirst AC source, typically an RF or microwave source. A first matchingnetwork is usually connected between the first AC source and thereactance for exciting the plasma. If the source is an RF source, thereactance is either a coil for supplying magnetic and electric fields tothe chamber interior via a dielectric window or a parallel platecapacitive arrangement for supplying an electric field to the chamberinterior.

[0003] The workpiece, which is typically a semiconductor wafer or adielectric sheet or a metal plate, is clamped in place on a workpieceholder, i.e., chuck, that frequently includes an electrode covered by adielectric. DC voltage is typically applied to the electrode to providean electrostatic clamping force to hold the workpiece in situ on theholder. The workpiece is usually cooled by applying a coolant agent,such as helium, to a recess in the chuck and by applying a liquid toconduits in the chuck. To accelerate ions in the plasma to theworkpiece, a second AC source is connected to the electrode by way of amatching network. Each matching network includes a pair of variablereactances having values that are varied by motors, typically stepmotors.

[0004] Sensors for electric parameters associated with the plasma, ascoupled to the excitation reactance and as coupled to the chuckelectrode, derive signals which assist in controlling the values of thevariable reactances. Pressure and flow rate transducers respectively inthe chamber and in a line supplying gas to the second port derivesignals which assist in controlling the vacuum pressure in the chamberand the flow rate of gas flowing into the chamber through the secondport.

[0005] A controller, including a microprocessor and a memory systemincluding a hard drive, random access memory (RAM) and a read onlymemory (ROM), responds to the signals derived by the transducers andsignals from an operator input console to produce signals forcontrolling the variable reactances, output power of the two AC sources,the vacuum pressure in the chamber and the flow rate of gases suppliedto the chamber through the second port. The memory system stores severalrecipes, each in the form of signals representing various parameterscontrolling the deposition and etching of the workpieces for differingsituations. The parameters of each recipe are, inter alia, gas speciesto be supplied to the chamber, flow rates of the species, vacuumpressure in the chamber and output powers of the two AC sources. Eachrecipe can include other parameters, such as time for carrying out eachrecipe step. The controller responds to the parameters of the recipe tocontrol valves for the flow of the gases into the chamber, the chamberpressure, as well as the output power of the first and second ACsources. During processing, the controller controls the reactances ofthe first and second matching networks so that there is an efficienttransfer of power between the first and second AC sources and the loadsthey drive so the impedances seen looking into the output terminals ofthe first and second sources are substantially equal to the impedancesthe first and second sources respectively see by looking from theiroutput terminals into cables connected to the first and second matchingnetworks.

[0006] Typically, a recipe change has been marked by step, i.e. sudden,changes in at least one of (1) gas flow rate, (2) chamber pressure, (3)power supplied to a plasma excitation coil, (4) the gas species flowinginto the chamber, and (5) power supplied to (a) an electrode, such asbottom electrode on which the workpiece is mounted or a top electrodefor exciting a gas to a plasma, or (b) an RF plasma excitation coil.These step changes result in sharp demarcations between layers etchedfrom the workpiece or deposited on the workpiece. For example, the stepchanges during etching of a trench in a workpiece, e.g., a siliconsubstrate, result in sharp corners between a wall and base of thetrench. Such step changes also frequently result in sharp corners at aboundary between a trench wall and a layer at the top of the trench.Such sharp corners can make it difficult to fill the trench duringsubsequent operations and have other known disadvantages, such ascausing stress related defects and/or electrical leakage.

[0007] One method of addressing the problem which has resulted insomewhat smooth transitions when certain recipe changes are madeinvolves adding dilutants, such as argon or helium, or passivationgases, such as oxygen, on a transient basis, to gases flowing into theprocessing chamber during a process recipe step occurs. However, thereare disadvantages in transiently adding dilutant and/or passivationgases to the processing chamber. Because of the relatively large volumeof a typical plasma processing chamber, a significant amount of time, upto ten seconds, is required to purge “old” gas from a line coupling gasfrom a gas source into the chamber. As a result, there are substantialincreases in workpiece processing time, to reduce chamber efficiency anddecrease workpiece throughput. In addition, changing the gas species ona transient basis results in a change in plasma impedance. The change inplasma impedance adversely affects the ability of the matching networkbetween the electric source and the coil and/or electrode to provide anefficient transfer of power between the source or sources and the drivenloads. In addition, the time for the new gas, i.e., the dilutant orpassivation gas, to flow into the chamber is likely to vary as afunction of gas line length between the chamber and the gas source. As aresult, precise control of the processing step is difficult to achieveand/or recipe processing steps must be customized for the different gasline lengths between the different gas sources and the chamber.

[0008] Chen et al, U.S. Pat. No. 5,807,789 discloses a method ofoperating a plasma processor to form in a semiconductor workpiece ashallow trench with a tapered profile and round corners. Such a shallowtrench is formed during successive recipe steps. During a first step theplasma power and chamber pressure are respectively relatively high andlow. During the next steps, the plasma power and chamber pressure arerespectively decreased and increased. The process continues in this wayfor at least one additional step.

[0009] In a particular etching embodiment Chen et al discloses, theplural gas species applied to the chamber remain the same and atconstant flow rates while power supplied to the plasma is reduced inthree steps, each of which occurs simultaneously with an increase inchamber pressure. During a first step, which lasts for eight seconds,the power supplied to a plasma excitation reactance and chamber pressureare respectively 800 watts and 50 millitorr. At the beginning of asecond eight second step, the supplied power is reduced suddenly from800 watts to 750 watts while chamber pressure is increased suddenly to80 millitorr. At the beginning of a third 46 second step, supplied poweris suddenly reduced further to 650 watts while chamber pressure issuddenly increased to 100 millitorr.

[0010] The aforementioned process suffers from similar problems to thepreviously mentioned problems associated with adding dilutants becauseof the substantial time required to change pressure in the relativelylarge volume vacuum chamber. In addition, the sudden power changesfrequently do not enable the corners to be rounded to the desiredextent.

[0011] It is, accordingly, an object of the invention to provide a newand improved method of and apparatus for operating a vacuum plasmaprocessing chamber.

[0012] An additional object of the invention is to provide a new andimproved method of and apparatus for operating a vacuum plasmaprocessing chamber in such a manner that sharp corners on processedworkpieces are avoided.

[0013] Another object of the invention is to provide a new and improvedmethod of and apparatus for controlling a vacuum plasma processor suchthat changes in a processing recipe are performed in a manner whichavoids sharp corners on a processed workpiece.

[0014] Still another object of the invention is to provide a new andimproved method of and apparatus for processing a workpiece in a vacuumplasma workpiece processor so that changes during a recipe are performedin such a way as to avoid sharp corners in a processed workpiece andwherein processor throughput is relatively high.

[0015] Still another object of the invention is to provide a new andimproved method of and apparatus for controlling processing ofworkpieces in a vacuum plasma processor, wherein changes in steps of arecipe are performed in such a way that sharp corners of the workpieceare avoided, without substantial impedance mismatches occurring betweenone or more sources driving reactive components which supply power toprocessing gas in the chamber.

SUMMARY OF THE INVENTION

[0016] In accordance with the present invention, AC power supplied to aplasma in a vacuum plasma workpiece processing chamber is controlled ona preprogrammed basis so there are gradual changes in the amount of ACpower supplied to the plasma during processing of one workpiece.Preferably, the gradual power change occurs while no change is made in(a) the gas species flowing into the chamber, (b) the chamber pressureor (c) the gas species flow rates. The AC power can be supplied to thechamber by an upper or lower chamber electrode coupling an AC electricfield to gas in the chamber or a coil coupling an AC electromagneticfield to the chamber gas. The gradual power change is typically suchthat it causes a gradual transition in the shape of material in theprocessed workpiece.

[0017] In one preferred embodiment, a gas species is ionized into aplasma that etches the material and the preprogrammed gradual powerchange and the species are such that the material is shaped so a roundedcorner is formed in the material as a result of the etching. In onespecific application, the etching forms a trench wall including therounded corner, which in one embodiment is at an intersection of a walland a base of a trench.

[0018] The gradual change is typically performed in response to acomputer program storing steps having (1) power changes in the range ofa few milliwatts to less than 5% of the maximum output power of a source(e.g., if a source has a maximum output power of 3 kW, the maximum powerchange is 15 watts), and (2) durations in the range of about 1millisecond to about 1 second. Steps having power changes greater thanabout 5% of maximum output power are too steep to provide the desiredcontrol over the plasma to achieve the desired workpiece shapes andsteps lasting longer than about 1 second do not have adequate temporalresolution to achieve the desired workpiece shapes.

[0019] The above and still further objects, features and advantages ofthe present invention will become apparent upon consideration of thefollowing detailed description of several specific embodiments thereof,especially when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

[0020]FIG. 1 is a block diagram of a typical vacuum plasma processor andcontroller capable of performing the present invention;

[0021]FIG. 2 is a waveform of power versus time that can be applied tothe coil or electrode of the apparatus illustrated in FIG. 1, whereinpower increases gradually in an upwardly ramping manner;

[0022]FIG. 3 is a waveform similar to the waveform of FIG. 2, whereinthe power ramps downwardly;

[0023]FIG. 4 is a waveform of power versus time that can be applied tothe coil and/or electrode of FIG. 1, wherein the waveform is derivedfrom experimental data;

[0024]FIG. 5 is a schematic diagram of a cross section of anillustrative semiconductor wafer prior to etching; and

[0025]FIG. 6 is an schematic diagram of the wafer illustrated in FIG. 5after it has been etched in accordance with a specific embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE DRAWING

[0026] The workpiece processor illustrated in FIG. 1 includes vacuumplasma processing chamber assembly 10, a first circuit 12 for driving areactance for exciting ionizable gas in chamber assembly 10 to a plasmastate, a second circuit 14 for applying RF bias to a workpiece holder inchamber assembly 10, and a controller arrangement 16 responsive tosensors for various parameters associated with chamber assembly 10 forderiving control signals for devices affecting the plasma in chamberassembly 10. Controller 16 includes microprocessor 20 which responds tovarious sensors associated with chamber assembly 10, as well as circuits12 and 14, and signals from operator input 22, which can be in the form,for example, of a keyboard. Microprocessor 20 is coupled with memorysystem 24 including hard disk 26, random access memory (RAM) 28 and readonly memory (ROM) 30. Microprocessor 20 responds to the various signalssupplied to it to drive display 32, which can be a typical computermonitor.

[0027] Hard disk 26 and ROM 30 store programs for controlling theoperation of microprocessor 20 and preset data associated with differentrecipes for the processes performed in chamber assembly 10. Thedifferent recipes concern gas species and flow rates applied to chamberassembly 10 during different processes, the output power of AC sourcesincluded in circuits 12 and 14, the vacuum applied to the interior ofchamber assembly 10, and initial values of variable reactances includedin matching networks of circuits 12 and 14.

[0028] Plasma chamber assembly 10 includes chamber 40 having metal,non-magnetic cylindrical side wall 42 and metal, non-magnetic base 44,both of which are electrically grounded. Dielectric, typically quartz,window 46 is fixedly positioned on the top edge of wall 42. Wall 42,base 44 and window 46 are rigidly connected to each other by suitablegaskets to enable a vacuum to be established within the interior ofchamber 40. Planar plasma excitation coil 48, for example, as configuredin Ogle, U.S. Pat. No. 4,948,458 or Holland et al., U.S. Pat. No.5,759,280, sits on or in very close proximity to the upper face ofwindow 46. Coil 48, an electric reactance, reactively supplies magneticand electric AC fields usually at an RF frequency such as 13.56 MHz, tothe interior of chamber 40, to excite ionizable gas in the chamber toplasma, schematically illustrated in FIG. 1 by reference numeral 50. Itis to be understood that for the purposes of the present invention, coil48 can be replaced with a powered or grounded electrode that extendsparallel to electrode 56 and can be located in chamber 40.

[0029] The upper face of base 44 carries holder, i.e. chuck, 52 forworkpiece 54, which is typically a circular semiconductor wafer, arectangular dielectric plate such as used in flat panel displays or ametal plate. Chuck holder 52 typically includes metal plate 56 thatforms an electrode (a reactive element). Electrode 56 carries dielectriclayer 58 and sits on dielectric layer 60, which is carried by the upperface of base 44. A workpiece handling mechanism (not shown) placesworkpiece 54 on the upper face of dielectric layer 58. Workpiece 54 iscooled by supplying helium from a suitable source 62 to the underside ofdielectric layer 58 via conduit 64 and grooves (not shown) in electrode56 and by supplying a liquid from a suitable source (not shown) toconduits (not shown) in chuck 52. With workpiece 54 in place ondielectric layer 58, DC source 66 supplies a suitable voltage through aswitch (not shown) to electrode 56 to clamp, i.e., chuck, workpiece 54to chuck 52.

[0030] With workpiece 54 secured in place on chuck 52, one or moreionizable gases from one or more sources 68 flow into the interior ofchamber 40 through conduit 70 and port 72 in sidewall 42. Forconvenience, only one gas source 68 is shown in FIG. 1, but it is to beunderstood that usually there are several gas sources of differentspecies, e.g. etchants, such as SF₆, CH₄, C₁₂ and HBr, dilutants such asAr or He, and O₂ as a passivation gas. The interior of conduit 70includes valve 74 and flow rate gauge 76 for respectively controllingthe flow rate of gas flowing through port 72 into chamber 40 andmeasuring the gas flow rate through port 72. Valve 74 responds to asignal microprocessor 20 derives, while gauge 76 supplies themicroprocessor with an electric signal indicative of the gas flow ratein conduit 70. Memory system 24 stores for each recipe step of eachworkpiece 54 processed in chamber 40 a signal indicative of desired gasflow rate in conduit 70. Microprocessor 20 responds to the signal memorysystem 24 stores for desired flow rate and the monitored flow ratesignal gauge 76 derives to control valve 74 accordingly.

[0031] Vacuum pump 80, connected to port 82 in base 44 of chamber 40 byconduit 84, evacuates the interior of the chamber to a suitablepressure, typically in the range of one to one hundred millitorr.Pressure gauge 86, in the interior of chamber 40, suppliesmicroprocessor 20 with a signal indicative of the vacuum pressure inchamber 40. Memory system 24 stores for each recipe step a signalindicative of desired vacuum pressure for the interior of chamber 40.Microprocessor 20 responds to the stored desired pressure signal memorysystem 24 derives for each recipe step and an electric signal frompressure gauge 86 to supply an electric signal to vacuum pump 80 tomaintain the pressure in chamber 40 at the set point or predeterminedvalue for each recipe step.

[0032] Optical spectrometer 90 monitors the optical emission of plasma50 by responding to optical energy emitted by the plasma and coupled tothe spectrometer via window 92 in side wall 42. Spectrometer 90 respondsto the optical energy emitted by plasma 50 to supply an electric signalto microprocessor 20. Microprocessor 20 responds to the signal thatspectrometer 90 derives to detect an end point of the process (eitheretching or deposition) that plasma 50 is performing on workpiece 54.Microprocessor 20 responds to the signal spectrometer 90 derives and asignal memory system 24 stores indicative of a characteristic of theoutput of the spectrometer associated with an end point to supply thememory with an appropriate signal to indicate that the recipe step hasbeen completed. Microprocessor 20 then responds to signals from memorysystem 24 to stop certain activities associated with the completedrecipe step and initiate a new recipe step on the workpiece beingprocessed in chamber 40 or commands release of workpiece 54 from chuck52 and transfer of a new workpiece to the chuck, followed by instigationof another series of recipe processing steps.

[0033] Excitation circuit 12 for driving coil 48 includes constantfrequency RF source 100, having a constant output power and typicallyhaving a frequency of 13.56 MHz. Source 100 drives power amplifier 102,having an electronically controlled power gain, so that the amplifierresponse time is on the order of a few microseconds or less, i.e., theoutput power of amplifier 102 changes from a first value to a secondvalue in a few microseconds or less. The output power of amplifier 102is in the range between 100 and 3000 watts. Amplifier 102 typically hasa 50 ohm output impedance all of which is resistive and none of which isreactive. Hence, the impedance seen looking back into the outputterminals of amplifier 102 is typically represented by (50+j0) ohms, andcable 106 is chosen to have a characteristic impedance of 50 ohms.

[0034] For any particular recipe, memory system 24 stores a signal fordesired output powers of amplifier 102. Memory system 24 supplies thedesired output power of amplifier 102 to the amplifier by way ofmicroprocessor 20. The output power of amplifier 102 can be controlledin an open loop manner in response to the signals stored in memorysystem 24 or control of the output power of amplifier 102 can be on aclosed loop feedback basis, as known in the art. The output power ofamplifier 102 is also gradually dynamically changed as a function oftime as preprogrammed changes in a recipe step are ordered by memorysystem 24. The preprogrammed dynamic changes in the output power arestored in memory system 24 and control the power gain of amplifier 102.

[0035] The output power of amplifier 102 drives coil 48 via cable 106and matching network 108. Matching network 108, typically configured asa “T,” includes two series legs including variable capacitors 112 and116, as well as a shunt leg including fixed capacitor 114. Coil 48includes input and output terminals 122 and 124, respectively connectedto one electrode of capacitor 112 and to a first electrode of seriescapacitor 126, having a grounded second electrode. The value ofcapacitor 126 is preferably selected as described in the commonlyassigned, previously mentioned, Holland et al. patent.

[0036] Electric motors 118 and 120, preferably of the step type, respondto signals from microprocessor 20 to control the values of capacitors112 and 116 in relatively small increments to maintain an impedancematch between the impedance seen by looking from the output terminals ofamplifier 102 into cable 106 and by looking from cable 106 into theoutput terminals of amplifier 102. Hence, for the previously described(50+j0) ohm output impedance of amplifier 102 and 50 ohm characteristicimpedance of cable 106, microprocessor 20 controls motors 118 and 120 sothe impedance seen looking from cable 106 into matching network 108 isas close as possible to (50+j0) ohms.

[0037] To control motors 118 and 120 to maintain a matched condition forthe impedance seen looking into the output terminals of amplifier 132and the impedance amplifier 132 drives, microprocessor 20 responds tosignals from conventional sensor arrangement 104 indicative of theimpedance seen looking from cable 106 into matching network 108.Alternatively, sensors can be provided for deriving signals indicativeof the power amplifier 102 supplies to its output terminals and thepower reflected by matching network 108 back to cable 106.Microprocessor 20 responds, in one of several known manners, to thesensed signals that sensor arrangement 104 derives to control motors 118and 120 to attain the matched condition.

[0038] Circuit 14 for supplying RF bias to workpiece 54 via electrode 56has a construction somewhat similar to circuit 12. Circuit 14 includesconstant frequency RF source 130, having a constant output power andtypically having a frequency such as 400 KHz, 2.0 MHz or 13.56 MHz. Theoutput of source 130 drives electronically controlled variable gainpower amplifier 132, having the same characteristics as amplifier 102.Amplifier 132 in turn drives a cascaded arrangement includingdirectional coupler 134, cable 136 and matching network 138. Matchingnetwork 138 includes a series leg comprising the series combination offixed inductor 140 and variable capacitor 142, as well as a shunt legincluding fixed inductor 144 and variable capacitor 146. Motors 148 and150, which are preferably step motors, vary the values of capacitors 142and 146, respectively, in response to signals from microprocessor 20.

[0039] Output terminal 152 of matching network 138 supplies an RF biasvoltage to electrode 56 by way of series coupling capacitor 154 whichisolates matching network 138 from the chucking voltage of DC source 66.The RF energy circuit 14 applies to electrode 56 is capacitively coupledvia dielectric layer 48, workpiece 54 and a plasma sheath between theworkpiece and plasma to a portion of plasma 50 in close proximity withchuck 52. The RF energy that chuck 52 couples to plasma 50 establishes aDC. bias in the plasma; the DC bias typically has values between 50 and1000 volts. The DC bias resulting from the RF energy circuit 14 appliesto electrode 52 accelerates ions in plasma 50 to workpiece 54.

[0040] Microprocessor 20 responds to signals indicative of the impedanceseen looking from cable 136 into matching network 138, as derived by aknown sensor arrangement 139, to control motors 148 and 150 and thevalues of capacitors 142 and 146 in a manner similar to that describedsupra with regard to control of capacitors 112 and 116 of matchingnetwork 108.

[0041] For each process recipe step, memory system 24 stores set pointsignals for the net power coupled by directional coupler 134 to cable136. The net power coupled by directional coupler 134 to cable 136equals the output power of amplifier 132 minus the power reflected fromthe load and matching network 138 back through cable 136 to theterminals of directional coupler 134 connected to cable 136. Memorysystem 24 supplies the net power set point signal associated withcircuit 14 to microprocessor 20. Microprocessor 34 also responds tooutput signals directional coupler 134 supplies to power sensorarrangement 141. Power sensor arrangement 141 derives signals indicativeof output power of amplifier 132 and power reflected by cable 136 backtoward the output terminals of amplifier 132.

[0042] Microprocessor 20 responds to the set points and measured signalssensor arrangement 141 derives, which measured signals are indicative ofthe output power of amplifier 132 and the power reflected back toamplifier, to control the power gain of amplifier 132. The output powerof amplifier 132 is also gradually dynamically changed as a function oftime as changes in a recipe are ordered by memory systems 24. Thedynamic changes in the output power are stored in memory system 24 andcontrol the power gain of amplifier 132.

[0043] One of the elements of memory system 24, typically read-onlymemory 30, stores preprogrammed values for controlling the output powerof amplifier 102 and/or 132 during a step of the recipe of plasma 50processing workpiece 54. The preprogrammed values thereby control theamount of power coil 48 and/or electrode 56 supply to the plasma 50 inchamber 40 to enable the power that coil 48 and/or electrode 56 suppliesto the plasma to change gradually as a function of time in accordancewith a preprogrammed predetermined function, such as the mathematicalfunctions 170 and 172 illustrated in FIGS. 2 and 3 or the empiricalfunction 174 illustrated in FIG. 4. Functions 170 and 172 of FIGS. 2 and3 are respectively upwardly and downwardly directed substantiallycontinuous, gradual linear ramping functions.

[0044] The preprogrammed values for controlling the output power ofamplifier 102 and/or 132 that read-only memory 30 stores are, inactuality, a series of relatively small incremental steps, each of whichusually has the same value. The incremental steps are such that theoutput power of amplifier 102 and/or 132 changes suddenly at thebeginning of each step, by a small value, in the range of about 1milliwatt to less than 5% of the maximum output power of amplifier 102and/or 132 (e.g., if the maximum output power of amplifier 102 is 3000watts, the maximum change in a step of the output power of amplifier is15 watts). Each step usually has the same relatively short duration,typically between one millisecond and one second during which the outputpower of amplifier 102 and/or 132 remains constant. Steps longer thanone second will not usually provide the desired rounding effectpreviously discussed in this document. A series of steps in theforegoing ranges provides substantially continuous and gradualvariations in power supplied to coil 48 and electrode 56 and thereforethe power supplied to plasma 50.

[0045] When it is desired to change the output power of amplifier 102and/or 132 in accordance with a preprogrammed function, such as thoseillustrated in any of FIGS. 2-4, the stored program in hard disk 26periodically reads stored numeric values in read-only memory 30indicative of the gain amplifier settings which provide the desiredoutput power of the amplifier. Microprocessor 20 responds to the valuesread from read-only memory 30 to control the gain of at least one ofamplifiers 102 and 132 and thereby vary the power at least one of coil48 and electrode 56 supplies to plasma 50.

[0046] For purposes of explanation, assume that the functions of FIGS.2, 3 and 4 represent the power RF source 30 and variable gain amplifier132 supply to electrode 56. Prior to the beginning of ramping function170, at time T1, memory system 24 and microprocessor 20 set the gain ofamplifier 132 so that electrode 56 supplies constant power P1 to plasma50. During a recipe step of interest, memory system 24 andmicroprocessor 20 control the gain of amplifier 132 to increase thepower supplied to electrode 56 as indicated by linear, upwardlydirected, gradually increasing and substantially continuous rampingfunction 170. Ramping function 170 continues until the recipe step hasbeen completed at time T2. Thereafter, memory system 24 andmicroprocessor 20 maintain the gain of amplifier 132 constant so thatelectrode 56 supplies constant power P2 to plasma 50. Memory system 24and microprocessor 20 control the gain of amplifier 132 and the powerelectrode 56 supplies to plasma 50 to gradually and substantiallycontinuously decrease the plasma power along ramping function 172.Ramping function 172 extends from a constant value P2, at time T1, to aconstant value P1, at time T2. The power decrease from P2 to P1 isperformed in the same manner described for upwardly directed rampingfunction 170.

[0047] The slopes of ramping functions 170 and 172 are determined by themagnitude and duration of each step change in the gain of amplifier 132.Typically, the magnitude and duration of each step change in the gain ofamplifier 132 for a particular recipe change are the same; it is to beunderstood, however, that different step changes in the gain ofamplifier 132 for a particular recipe change can have differentmagnitude and duration values.

[0048] Function 174 of FIG. 4 is empirically derived and results from aseries of experiments performed on test workpieces 54 under various RFpower settings and from measurements of profile angles under thesevarious power settings. It is to be understood that function 174 of FIG.4 is merely for illustrative purposes and that many differentempirically derived functions can be employed, as necessary. Theparticular function 174 varies between a constant power level P3 to ahigher constant power level P4, which respectively subsist prior to timeT3 and subsequent to time T4, the temporal boundaries of function 174.Function 174 decreases slowly from power level P3, then increases at afaster rate to a value above power level P4 and then returns to powerlevel P4 at time T4.

[0049] Reference is now made to FIGS. 5 and 6 of the drawing,respectively schematic drawings of an illustrative semiconductorstructure prior and subsequent to etching operations in accordance withone embodiment of the present invention. The pre-etch structure of FIG.5 includes silicon substrate 202 having a top face coated by thin filmsilicon oxide layer 204, typically having a thickness of 150 angstroms,which in turn is covered by a thin film silicon nitride layer 206,typically having a thickness of 1600 angstroms. Layer 206 is coated withan epitaxial bottom anti-reflective coating 208, typically having athickness of 570 angstroms, in turn covered by two spaced photoresiststrips 210.

[0050] The structure of FIG. 5 is initially processed by reducing theheight of photoresist strips 210 to form truncated photoresist strips212. Photoresist strips 210 are reduced in height by supplying a typicalphotoresist etchant from gas sources 68 to the interior of chamber 40under the control of a program hard disk 26 stores and which is read tomicroprocessor 20. Simultaneously, the program that hard disk 26 storesand microprocessor 20 cause the power supplied to coil 48 and electrode56 and the vacuum in chamber 40 to remain constant. Next, the programthat hard disk 26 stores causes bottom anti-reflective coating 28 to beetched by opening valves 74 connected to hydrogen bromide (HBr) andoxygen (O₂) sources 68 so that the flow ratio of these gases is 75 to22. At the same time, disk 26 and microprocessor 20 control vacuum pump80 so the pressure in chamber 40 is 3 millitorr. Simultaneously, harddisk 26 causes the gains of amplifiers 102 and 132 to be such that coil40 is supplied with 500 watts of RF power at 13.56 MHz while electrode56 is supplied with 178 watts of RF power at 13.56 MHz; the 178 watts ofRF power supplied to electrode 56 causes a DC bias of −200V to beestablished on the electrode. Optical spectrometer 90 detects when theetching end point of layer 208 occurs. Microprocessor 20 and memorysystem 24 respond to the signal from optical spectrometer 90 to causeover etching of layer 208 by 30 percent, a result achieved by notchanging the etching parameters in chamber 40.

[0051] When the over etch has been completed, microprocessor 20 andmemory system 24 cause silicon nitride layer 206 to be etched in asomewhat similar matter to that described in connection with coating 208until optical spectrometer 90 detects an etch end point. Etching oflayer 206 is in response to a suitable mixture of fluorine-basedetchants while the chamber 40 pressure is 10 millitorr, the RF powersupplied to coil 48 is 1000 watts and the RF power supplied to electrode56 is 155 watts, resulting in the electrode having a DC bias of −70V.

[0052] Microprocessor 20 and memory system 24 then cause silicon nitridelayer 206 to be over etched for 10 seconds. The over etch is performedby causing a suitable mixture of fluorine-based etchants argon andoxygen to flow from sources 68 into chamber 40, while the chamber ismaintained at a pressure of 7 millitorr, and 1400 and 400 watts RF powerare respectively applied to coil 48 and electrode 56. The application of400 watts RF power to electrode 56 results in the electrode being at aDC voltage of −145 V.

[0053] Microprocessor 20 and memory system 24 then cause a breakthroughof silicon oxide layer 204 by causing 100 sccm Cl₂ to be applied forfive seconds from gas sources 68 to chamber 40, while 500 and 120 wattsare respectively applied to coil 48 and electrode 56.

[0054] Microprocessor 20 and memory system 24 then cause the main etchoperation for shallow trench isolation of silicon substrate 202 to beperformed. The main etch operation is performed for 65 seconds inresponse to a suitable mixture of HBr/Cl₂/O₂ flowing from gas sources 68to chamber 40. During the 65 seconds, vacuum pump 80 maintains thepressure in chamber 40 at 15 millitorr, the output of amplifier 102supplies coil 48 with 1000 watts and amplifier 132 supplies electrode 56with 235 watts so that the DC bias voltage of electrode 56 is −320V.

[0055] Upon completion of the 65 second main etch operation, the siliconin substrate 202 is at the location indicated by point 212, FIG. 7,slightly above the trench final base 214. The final etch operation ofsilicon substrate 202 between point 212 and base 214 is performed insuch a manner as to achieve rounded edges 216 between point 212 and base214.

[0056] To this end, microprocessor 20 and memory system 24 perform thefinal etch operation for 15 seconds. During the 15 second final etchoperation, vacuum pump 80 maintains the pressure in chamber 40 constantat 10 millitorr, amplifier 102 maintains the power supplied to coil 48constant at 100 watts and a suitable mixture of HBr/O₂ constantly flowsfrom sources 68 into chamber 40, while the power that amplifier 132supplies to electrode 56 gradually decreases from 200 to 100 watts. Thegradual decrease in the power that amplifier 132 supplies to electrode56 is in 15,000 steps, each having a duration of 1.0 millisecond and anamplitude of 6.667 milliwatts. After base 214 has been reached, theetchant gases are purged from the chamber while the chamber pressureremains constant as does the power supplied to coil 48 and electrode 56.Then workpiece 54 can be removed from chamber 40 for further processing.

[0057] If it is desired to provide a rounded corner, that is a gradualtransition, between the top portion of silicon substrate 202 and a layerdeposited thereon, the power applied to electrode 56 and/or coil 48 canbe similarly varied while the gas species and the flow rates thereofinto chamber 40 are maintained constant, simultaneously with thepressure in chamber 40 remaining constant. Gradual transitions indeposited layers can also be provided by gradually and substantiallycontinuously varying the power applied to coil 48 and/or electrode 56,while maintaining constant the gas species, the flow rates thereof andthe pressure in chamber 40. Because of the fast response times ofamplifiers 102 and 132, the changes in the gains of these amplifiersalmost instantaneously change the characteristics of plasma 50 toprovide relatively high throughput processing of workpieces 54 and moreaccurate control of workpiece processing during a recipe step changethan can be provided by varying a parameter such as gas flow rate orchamber pressure.

[0058] While there have been described and illustrated specificembodiments of the invention, it will be clear that variations in thedetails of the embodiments specifically illustrated and described may bemade without departing from the true spirit and scope of the inventionas defined in the appended claims. For example, the variable gainfeatures provided by amplifiers 102 and 132 can be incorporated directlyinto RF sources 100 and 30, respectively.

We claim:
 1. A method of processing a workpiece in a vacuum plasmaprocessor chamber wherein a gas species is converted into an AC plasma,the vacuum chamber being subject to operating at different pressureswhile the workpiece is being processed, the gas species being subject toflowing into the chamber at different flow rates while the workpiece isbeing processed, comprising, gradually changing on a pre-programmedbasis, the amount of AC power supplied to the plasma during processingof the workpiece.
 2. The method of claim 1 wherein the gradual powerchange occurs while no change is made in (a) the species, (b) thepressure or (c) the flow rate.
 3. The method of claim 1 wherein the ACpower is supplied by an electrode coupling an AC electric field toplasma in the chamber.
 4. The method of claim 3 wherein the electrode isresponsive to an AC power source that supplies RF bias voltage to theelectrode, the electrode being on a holder for the workpiece.
 5. Themethod of claim 3 wherein the electrode is responsive to an AC powersource that supplies RF plasma excitation voltage to the electrode, theelectrode responding to the RF voltage to supply RF electric field tothe plasma to excite the gas to the plasma.
 6. The method of claim 3wherein the AC power is supplied by a coil coupling an RF plasmaexcitation electromagnetic field to the chamber.
 7. The method of claim1 wherein a gradual transition in the shape of material in the workpiecebeing processed occurs in response to the gradual power change.
 8. Themethod of claim 7 wherein the species is ionized into a plasma thatetches the material, the gradual power change and the species being suchthat the material is shaped to have a rounded corner in response tochanges in the ionized plasma etchant resulting from the gradual powerchange.
 9. The method of claim 8 wherein the etching, which occurs inresponse to changes in the ionized plasma etchant resulting from thegradual power change, forms a trench wall including the rounded corner.10. The method of claim 9 wherein the rounded corner is at anintersection of a wall and a base of a trench.
 11. The method of claim 7wherein the rounded corner is at an intersection of a wall and a surfaceintersecting the wall, the surface extending generally at right anglesto the wall.
 12. The method of claim 1 wherein the gradual changeincludes steps having power changes no greater than about several watts,the power remaining at a constant wattage for no more than about 1second.
 13. The method of claim 12 wherein the power steps are a fewmilliwatts and remain at a constant power for about 1 millisecond.
 14. Avacuum plasma processor for processing a workpiece in a vacuum plasmaprocessor chamber wherein a gas species is converted into an AC plasmacomprising a reactive element for supplying an electric field to plasmain the chamber, and an electric source for supplying gradually changingamounts of power on a preprogrammed basis to the reactive element. 15.The processor of claim 14 further including a controller for causing thesource to supply the gradually changing amounts of power on thepreprogrammed basis to the reactive element while a single workpiece isbeing processed.
 16. The processor of claim 15 wherein the controller isarranged for (a) controlling (i) a gas species adapted to flow into thechamber, (ii) the pressure in the vacuum chamber, and (iii) the flowrates of the gas species, and (b) maintaining the constant (I) the gasspecies, (ii) the gas species flow rate and (iii) the chamber pressurewhile the plasma power is gradually changing on the preprogrammed basis.17. A computer program for controlling a computer for controllingprocessing of a workpiece in a vacuum plasma processor chamber wherein agas species is converted into an AC plasma, the computer program storingsignals causing (a) the vacuum chamber to operate at different pressureswhile the workpiece is being processed, (b) control of the gas speciestype and the flow rates thereof into the chamber while the workpiece isbeing processed, (c) the amount of AC power applied to the plasma whilethe workpiece is being processed; the stored signal for the amount ofapplied AC power causing gradual preprogrammed changes in the amount ofAC power supplied to the plasma during processing of the workpiece. 18.The program of claim 17 wherein the stored signal causes the gradualpower change to occur while no change is made in (a) the species, (b)the pressure or (c) the flow rate.
 19. The program of claim 17 whereinthe stored signal causing gradual power change causes a gradualtransition in the shape of material in the workpiece being processed inresponse to the gradual power change.
 20. The program of claim 19wherein the stored signal controls etchant species supplied to thechamber while the workpiece is being processed and the gradual powertransition so as to cause the workpiece to be etched to have a roundedcorner.
 21. The program of claim 20 wherein the stored signal controlsetchant species supplied to the chamber while the workpiece is beingprocessed and the gradual power transition so as to cause the workpieceto be etched to have a trench wall including the rounded corner.
 22. Theprogram of claim 21 wherein the rounded corner is at an intersection ofa wall and a base of a trench.